Diet foods and artificial sweeteners may trigger weight gain and increase the risk of diabetes, as their sweet taste fools the body's metabolism into believing that we are consuming more calories, scientists say.

In nature, sweetness signals the presence of energy and its intensity reflects the amount of energy present.When a beverage is either too sweet or not sweet enough for the amount of calories it contains, the metabolic response and the signal that communicates nutritional value to the brain are disrupted, according to researchers from Yale University in the US.

A sweet-tasting, lower-calorie drink can trigger a greater metabolic response than drinks with higher calories, explaining the association between artificial sweeteners and diabetes discovered in earlier studies, researchers said.

The study, published in the journal Current Biology, shows that sweetness helps to determine how calories are metabolised and signalled to the brain.When sweetness and calories are matched, the calories are metabolised, and this is registered by the brains reward circuits.

However, when a mismatch occurs, the calories fail to trigger the body's metabolism and the reward circuits in the brain fail to register that calories have been consumed.

"In other words, the assumption that more calories trigger greater metabolic and brain response is wrong," said Dana Small, professor at Yale University. "Calories are only half of the equation; sweet taste perception is the other half," Small said.

Small noted that many processed foods contain such mismatches - such as a yogurt with low calorie sweeteners. "Our bodies evolved to efficiently use the energy sources available in nature," Small said. "Our modern food environment is characterised by energy sources our bodies have never seen before," she added.